1. Field of the Invention
The present invention relates to a method of diffusing an impurity into a semiconductor wafer.
2. Description of the Prior Art
In diffusing an impurity into semiconductor wafers, for example, in diffusing a P-type impurity into silicon wafers by the use of boron nitride (BN), the following is a description of a known method. Wafers (partially oxidized to some extent) of boron nitride (which is a P-type impurity source) and wafers of silicon are alternately arranged on a boat. In this arrangement, the boat is inserted into a silica tube. Thereafter, the silica tube is heated from an outer periphery, to form a boron oxide atmosphere inside the silica tube. In this manner, boron oxide is deposited on the silicon wafers, and besides, the impurity oxide is diffused into them. In this case, an inert gas (N.sub.2) is fed in through one end of the silica tube, so as to prevent the surfaces of the silicon wafers from being oxidized.
With this known method, however, the silica tube is in the open state during the heating and the inert gas is fed in through one end of the silica tube, so that the temperature distribution within the silica tube is not uniform in its lengthwise direction. For this reason, the quantity of impurity introduced into the silicon wafers lined up on the boat is dispersed, i.e., varied, and accordingly, surface specific resistance of the silicon wafers depends on the lined-up position of each wafer.
This point will now be described in detail. As stated above, the inert gas is caused to flow in through one end of the silica tube. Even when the inflowing inert gas reaches a uniformly heated portion within the tube, it does not yet arrive at the uniform heating temperature. Therefore, the inert gas lowers the temperature of the boron nitride and silicon wafers at the uniformly heated portion of the tube. In addition, the inert gas disturbs the flow of a boron oxide impurity gas produced from the boron nitride. Further, the other end of the silica tube is open in the foregoing boron deposition and diffusion process. Consequently, the air intrudes into the tube from the open end, oxidizes the surfaces of the silicon wafers and locally checks the deposition of impurity. Due to such various factors, the quantity of impurity introduction into the silicon wafers varies in dependence on the lined-up position of the wafers on the boat, and a variation in the surface specific resistance arises.
As a method for compensating for the drawbacks described above, the so-called ampoule diffusion procedure is contemplated in which silicon wafers and an impurity are sealed in a vacuum ampoule, the ampoule is inserted into a silica tube, and the tube is heated to directly vaporize atoms of the impurity and to diffuse them into the semiconductor wafers. With this method, however, the semiconductor surface becomes rough. In addition, it is necessary to especially fabricate the ampoule and to break it after completion of the diffusion. Thus, the method is uneconomical and is troublesome in procedure.